The present invention relates to a method for measurement of magnetic fields, a method for production of gradient coils, a gradient coil and an apparatus for magnetic resonance imaging, and more particularly to a method for measuring, after stopping application of a gradient magnetic field, the residual magnetic field; a gradient coil whose higher term of residual magnetic field is smaller; a manufacturing method for the same; and an apparatus for magnetic resonance imaging having such a gradient coil.
In a magnetic resonance imaging (MRI) apparatus, a subject of imaging is brought into the bore of a magnet system, i.e. a space in which a magnetostatic field is formed, to generate magnetic resonance signals in the subject by applying gradient magnetic fields and high frequency magnetic fields, and sectional images are generated (reconstructed) on the basis of received magnetic resonance signals.
In a magnet system which uses permanent magnets for generating a magnetostatic field, a pole piece for uniformizing the magnetic flux distribution in the magnetostatic field space is provided at the tip of each of paired permanent magnets opposite to each other, and a gradient coil for generating gradient magnetic fields is provided along the pole face of each such pole piece.
In the magnet system described above, as each gradient coil is close to a pole piece, the pole piece is magnetized by the gradient magnetic field, and the phase of the spin is affected by a magnetic field formed by its residual magnetization as if there were an eddy current having an extremely long time constant. As a result, imaging that calls for precise phase control, such as that by a fast spin echo (FSE) method is disturbed.
So far, because of the absence of an appropriate method to measure magnetic fields attributable to residual magnetization, it has been impossible to work out a way to eliminate its impact.
Therefore, an object of the present invention is to realize a method for measuring, after stopping application of gradient magnetic fields, the residual magnetic fields; a method for manufacturing a gradient coil by causing the characteristics of the residual magnetic fields, revealed by the measurement, to be reflected in it; a gradient coil manufactured by that method; and an apparatus for magnetic resonance imaging having such a gradient coil.
(1) In order to solve the above-noted problem, the invention from one point of view is a method for measurement of magnetic fields characterized in that a magnetically resonant sample is arranged at a measuring point on the surface of an imaginary sphere in a measuring space; a gradient magnetic field is applied to the measuring space; RF excitation is carried out after stopping the application of the gradient magnetic field to measure FID signals generated by the sample; the magnetic field intensity is calculated on the basis of a difference in differentials of the phases of the FID signals; and a magnetic field intensity is fitted into a spheric surface function representing a magnetic field intensity distribution in the measuring space.
According to the invention from this point of view, a spheric surface function representing the characteristics of the residual magnetic field is identified by measuring the magnetic field in which residual magnetization arises by utilizing FID signals from a sample arranged at a measuring point on a spheric surface and fitting the measurement into the spheric surface function.
(2) In order to solve the above-noted problem, the invention from another point of view is a method for measurement of magnetic fields characterized in that a magnetically resonant sample is arranged at a measuring point on the surface of an imaginary sphere in a measuring space; another magnetically resonant sample is arranged at the center of the sphere; a gradient magnetic field is applied to the measuring space; RF excitation is carried out after stopping the application of the gradient magnetic field to measure FID signals generated by the samples; a magnetic field intensity at the measuring point is calculated on the basis of a difference in differentials of the phases of FID signals generated by the samples at the measuring point and at the center of the sphere; and the magnetic field intensity is fitted into a spheric surface function representing a magnetic field intensity distribution in the measuring space.
According to the invention from this point of view, FID signals from a sample arranged at the center of the sphere are measured and used as reference.
(3) In order to solve the above-noted problem, the invention from still another point of view is a method for measurement of magnetic fields characterized in that: a magnetically resonant sample is arranged at a measuring point on the surface of an imaginary sphere in a measuring space; a gradient magnetic field is intermittently applied while the gradient is successively varied from the maximum gradient in one polarity to the maximum gradient in the reverse polarity and then the gradient is successively varied from the maximum gradient in the reverse polarity to the maximum gradient in the first polarity; RF excitation is carried out during the intermittence of the gradient magnetic field to measure FID signals generated by the sample; differentials of the phases of the FID signals are calculated; a magnetic field intensity at the measuring point is calculated on the basis of the hysteresis of differences in the differentials accompanying the completion of the round of gradients; and the magnetic field intensity is fitted into a spheric surface function representing a magnetic field intensity distribution in the measuring space.
According to the invention from this point of view, the gradient magnetic field is varied reciprocatingly between the maximum gradient in one polarity and the maximum gradient in the reverse polarity. This makes possible magnetic field measurement with the hysteresis of residual magnetization kept constant.
(4) In order to solve the above-noted problem, the invention from still another point of view is a method for measurement of magnetic fields characterized in that: a magnetically resonant sample is arranged at a measuring point on the surface of an imaginary sphere in a measuring space; another magnetically resonant sample is arranged at the center of the sphere; a gradient magnetic field is intermittently applied while the gradient is successively varied from the maximum gradient in one polarity to the maximum gradient in the reverse polarity and then the gradient is successively varied from the maximum gradient in the reverse polarity to the maximum gradient in the first polarity; RF excitation is carried out during the intermittence of the gradient magnetic field to measure FID signals generated by the samples; differences in differentials of the phases of FID signals generated by the samples at the measuring point and at the center of the sphere are calculated; a magnetic field intensity at the measuring point is calculated on the basis of the hysteresis of the differences in the differentials accompanying the completion of the round of gradients; and the magnetic field intensity is fitted into a spheric surface function representing a magnetic field intensity distribution in the measuring space.
According to the invention from this point of view, FID signals from a sample arranged at the center of the sphere are measured and used as reference.
(5) In order to solve the above-noted problem, the invention from still another point of view is the method for measurement of magnetic fields, according to any of (1) through (4), characterized in that: the measurement is consecutively accomplished at a plurality of measuring points.
According to the invention from this point of view, the same sample can be used repeatedly because measurement is done consecutively at a plurality of measuring points.
(6) In order to solve the above-noted problem, the invention from still another point of view is a method for production of gradient coils characterized in that, in producing a gradient coil: a magnetic field to be generated by the gradient coil on the surface of an imaginary sphere in a space is calculated; the magnetic field is fitted into a spheric surface function; and the current pass of the coil is determined so as to keep small, out of high order terms in the spheric surface function into which the fitting has been carried out, a high order term corresponding to the highest order term in the spheric surface function representing the magnetic field measured by a method according to any of (1) through (5).
According to the invention from this point of view, a gradient coil which generates less affecting residual magnetization can be produced because the current pass of the gradient coil is determined so as to keep small a high order term corresponding to the highest order term of the residual magnetic field revealed by measurement.
(7) In order to solve the above-noted problem, the invention from still another point of view is a gradient coil for generating a gradient magnetic field with a current flowing through a current pass, the gradient coil being characterized in that,
the current pass is determined by the following procedure:
(a) a magnetic field to be generated by the gradient coil on the surface of an imaginary sphere in a space is calculated;
(b) the magnetic field is fitted into a spheric surface function; and
(c) the current pass of the coil is determined so as to keep small, out of high order terms in the spheric surface function into which the fitting has been carried out, a high order term corresponding to the highest order term in the spheric surface function representing the magnetic field measured by a method according to any one of (1) through (5).
The invention from this point of view results in a gradient coil which generates less affecting residual magnetization can be produced because the current pass of the gradient coil is determined so as to keep small a high order term corresponding to the highest order term of the residual magnetic field revealed by measurement.
(8) In order to solve the above-noted problem, the invention from still another point of view is an apparatus for magnetic resonance imaging for structuring an image on the basis of magnetic resonance signals acquired by using a magnetostatic field, gradient magnetic fields and high frequency magnetic fields, the apparatus for magnetic resonance imaging being characterized in that, it is provided with, as a gradient coil for generating the gradient magnetic field, a gradient coil having a current pass determined by the following procedure:
(a) a magnetic field to be generated by the gradient coil on the surface of an imaginary sphere in a space is calculated;
(b) the magnetic field is fitted into a spheric surface function; and
(c) the current pass of the coil is determined so as to keep small, out of high order terms in the spheric surface function into which the fitting has been carried out, a high order term corresponding to the highest order term in the spheric surface function representing the magnetic field measured by a method according to any one of (1) through (5).
According to the invention from this point of view, appropriate magnetic resonance imaging can be accomplished using a gradient coil which generates less affecting residual magnetization because the current pass of the gradient coil is determined so as to keep small a high order term corresponding to the highest order term of the residual magnetic field revealed by measurement.
Therefore, the present invention makes it possible to realize a method for measuring, after stopping application of gradient magnetic fields, the residual magnetic fields; a method for manufacturing gradient coils reflecting the characteristics of residual magnetic fields revealed by the measurement; a gradient coil manufactured by the method; and an apparatus for magnetic resonance imaging having such gradient coils.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.